Conversion of aliphatic compounds to aromatics is an ongoing area of study for chemical manufacture. Certain aromatic compounds, such as para-xylene, have a relatively high commercial value. Processes that can allow formation of aromatics from feed streams that primarily have fuel value, such as natural gas or fuel gas, can be beneficial if the conversion to aromatics can be performed at a reasonable cost. Due to the relatively low activity of alkane compounds in processes for direct conversion to aromatics, additional improvements in processes for conversion of alkane to aromatics are desirable.
U.S. Pat. No. 5,043,502 describes a method for dehydroaromatization of C2-C5 aliphatic hydrocarbons to form aromatics. Para-xylene is produced both from the dehydroaromatization reaction and from subsequent methylation of toluene generated by the dehydroaromatization reaction. Benzene is also produced from dehydroaromatization and is described as being methylated to toluene during the subsequent methylation.
Other conventional processes can utilize feeds containing methane. Although methane is abundant, its relative inertness has limited its utility in these conversion processes. For example, oxidative coupling methods generally involve highly exothermic methane combustion reactions, frequently require expensive oxygen generation facilities, and produce large quantities of low value carbon oxides. Non-oxidative methane aromatization is equilibrium-limited, and temperatures ≥about 800° C. are needed for methane conversions greater than a few percent.
To obviate these problems, catalytic processes have been proposed for co-converting methane and one or more co-reactants to higher hydrocarbon, such as aromatics. For example, U.S. Pat. No. 5,936,135 discloses reacting methane at a temperature in the range of 300° C. to 600° C. with (i) a C2-10 olefin and/or (ii) a C2-10 paraffin in the presence of a bifunctional pentasil zeolite catalyst, having strong dehydrogenation and acid sites, to produce aromatics. The preferred mole ratio of olefin and/or higher paraffin to methane and/or ethane in the feed ranges from about 0.2 to about 2.0.
Other processes utilize organic oxygenate as a co-reactant for the non-oxidative methane conversion to produce higher hydrocarbon, including aromatics. For example, U.S. Pat. No. 7,022,888 discloses a process for the non-oxidative conversion of methane simultaneously with the conversion of an organic oxygenate, represented by a general formula: CnH2n+1OCmH2m+1, wherein C, H and O are carbon, hydrogen and oxygen, respectively; n is an integer having a value between 1 and 4; and m is an integer having a value between zero and 4. However, since these co-reactants are themselves valuable commodities, there is interest in developing alternative routes for the conversion of methane and/or other low molecular weight hydrocarbons into aromatics, particularly via routes that allow incorporation of more feed hydrocarbon into the aromatic product. It is desirable for such conversion reactions to operate over a broad molar ratio range of methane to other hydrocarbons in the feed.